Nag that's unhappy with saddle over its middle (5)
I believe the answer is:
scold
'nag' is the definition.
(I know that nag can be written as scold)
'unhappy with saddle over its middle' is the wordplay.
I cannot quite understand how this works, but
'saddle' could be 'col' (I've seen this before) and 'col' is found in the answer.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for scold that I've seen before include "Chide - nag" , "Censure, berate" , "Complaining woman" , "Angrily rebuke" , "Chide - fishwife" .)